1. Field of the Invention
The present invention relates generally to an apparatus and method for reducing detection complexity in a Multiple Input Multiple Output (MIMO) antenna system. More particularly, the present invention relates to an apparatus and method for lowering tree searching complexity of a receiver in a MIMO antenna system using a spatial multiplexing scheme.
2. Description of the Related Art
Recently, the wireless mobile communication market has grown rapidly. As the market has grown, demand has increased for various multimedia services in the wireless environment. In particular, demand is rising for increased mass transmission data capacity and rapid data delivery. In response to this rising demand, there is a need to find a method of efficiently using the limited frequencies. One proposed response is the use of a new transmission technique using a multi-antenna. By way of example of the new transmission technique, a Multiple Input Multiple Output (MIMO) system using a multi-antenna is being used.
A MIMO technique, which uses a multi-antenna at a transmitter and a multi-antenna at a receiver, can increase the channel transmission capacity in proportion to the number of the antennas. Such a technique increases the channel transmission capacity without using additional frequencies and without requiring additional transmit power allocation, as compared to a system using a single antenna. Thus, in recent years, active research has been conducted on the MIMO technique.
Multi-antenna techniques are divided largely into a spatial diversity scheme, a Spatial Multiplexing scheme and a combination scheme. The spatial diversity scheme improves the transmission reliability by acquiring a diversity gain corresponding to the product of the number of transmit antennas and the number of receive antennas. The Spatial Multiplexing (SM) scheme increases the data rate by transmitting a plurality of signal streams at the same time. The combination scheme is a combination of the spatial diversity and the SM schemes.
In the Vertical Bell Labs Layered Space-Time (V-BLAST) scheme, which is a representative SM scheme, a transmitter sends independent data on a plurality of antennas. Generally, when a receiver in a V-BLAST system adopts a Maximum Likelihood (ML) detection scheme, optimal performance can be obtained. However, the ML detection scheme has an extremely high complexity and therefore hinders or even disables its implementation. As alternatives to the ML detection scheme, a chase coding scheme and a QR Decomposition based M (QRD-M) scheme have been suggested. These schemes are less complex than the ML scheme yet approach the performance of the ML scheme. However, those systems are still highly complex and also face difficulty in being applied to an actual system.
The chase decoding scheme is a detection scheme which combines an Ordered Successive Interference Cancellation (OSIC) scheme and a List scheme. After first determining a signal to be detected, the chase decoding successively rejects interference by generating a list of the determined signal by the modulation order. A transmit vector is generated by the modulation order at each stage. A distance is measured between the transmit vectors and the received signal and the transmit signal is determined as the shortest vector between the receive signal and the transmit vectors.
The QRD-M scheme generates a triangular tree through the QR decomposition on the channel. The number of tree levels corresponds to the number of transmit antennas, and the number of branches extendible from a point of the level to a next level is determined by the modulation order. The QRD-M scheme searches the tree down to the lowest level while leaving M-ary branches at each level. When the M value is equal to the modulation order used in the system, performance close to the ML scheme is attained.
As discussed above, since the SM system, such as V-BLAST, transmits the independent data in parallel, the structure of the transmitter is quite simple. However, if the receiver detects the signals using the ML scheme, a Euclidean distance (or Euclidean metric) for MNt-ary vectors (M is the modulation order and Nt the number of antennas of the transmitter) needs to be computed. Because the complexity of the computation increases exponentially, its actual implementation is substantially impossible. In the chase decoding scheme and the QRD-M scheme, which detect the signal with lower complexity than the ML scheme, the list size and the M value need to be the same as the modulation order of the transmitter to acquire the ML performance. In this case, complexity makes their implementation impossible as well.
Accordingly, when using an SM scheme, the system requires low complexity and performance close to the ML scheme.